1. Nervous System
Some activities that require activity from the nervous system:
Feeling
Thinking
Remembering
Moving
Being aware of the world around you
Helps coordinate all other body functions to maintain homeostasis
Enables the body to respond to changing conditions
Information from within and outside the body is brought to the brain and spinal cord, which then stimulate responses from muscles and glands.

2. Nervous System
Nervous tissue consists of masses of nerve cells, or neurons.
Structural and functional units of the nervous system,
Specialized to react to physical and chemical changes in their surroundings
Transmit information in the form of electrochemical changes called nerve impulses
Transmit nerve impulses to other neurons and to cells outside the nervous system
Do not divide (no mitosis)

3. Nervous System Characteristics of neurons:
Have a rounded area called the cell body or soma
Contains the nucleus
Have two types of extensions:
Dendrites
May be numerous
Receive electrochemical messages
Axons
Send information in the form of nerve impulses
Each neuron usually has only one axon
Nerves are bundles of axons

4. Nervous System Neuroglia or neuroglial cells
Cells also found in the nervous system
Provide physical support to neurons
Provide insulation to neurons
Provide nutrients for neurons
Release and relay signals to guide differentiation of neurons prior to birth

5. Divisions of the Nervous System
The organs of the nervous system can be divided into two major subdivisions:
Central nervous system (CNS)
Consists of the brain and spinal cord
Peripheral nervous system (PNS)
Consists of nerves that connect the central nervous system to other body parts
Together these provide three general functions:
Sensory
Integrative
Motor

6. Sensory Function
Comes from sensory receptors that gather information by monitoring
External environmental factors such as light and sound intensities
Conditions of the internal environment such as temperature and oxygen levels
Sensory receptors convert environmental information into nerve impulses which are sent to the CNS.

7. Sensory Organs

8. Integrative Function
The information from sensory neurons are converted into nerve impulses which are
Transmitted over peripheral nerves to the central nervous system, and
Integrated or brought together:
Creating sensations
Adding to memory
Helping to produce thoughts
Translating sensations into perceptions

9. Integrative Function

10. Motor Function As a result of the integrative functions,
We make conscious or subconscious decisions
Use our motor functions to act on our decisions.
Motor functions employ peripheral neurons
Carry impulses from the CNS to responsive structures called effectors which
Are outside the nervous system
Include muscles that contract and glands that secrete when stimulated

11. Motor Function
The motor functions of the peripheral nervous system can be divided into two categories
Somatic nervous system
Consciously controlled
Controls skeletal muscles
Autonomic nervous system
Involuntarily controlled
Controls effectors that are involuntary such as the heart and smooth muscles
The nervous system maintains homeostasis by responding to internal or external changes in environment.

12. Motor Function

13. Neuroglial Cells (Neuroglia)
Greatly outnumber neurons
Can divide (carry on mitosis)
Can produce fast-growing brain tumors called gliomas
Functions of neuroglia:
Help neurons survive by:
Filling spaces
Providing structural frameworks
Producing components of myelin (electrical insulator)
Providing nutrients for neurons
Carrying on phagocytosis

14. Types of Neuroglia Microglial cells Oligodendrocytes
Tiny cells found only in the CNS
Scattered throughout the CNS
Support neurons
Phagocytize bacterial cells and cellular debris
Oligodendrocytes
Align along nerve fibers only in the CNS
Provide insulating layers of myelin called a myelin sheath found around axons within the CNS
Only source of myelin in the CNS

15. Types of Neuroglia Astrocytes
Star-shaped neuroglia found only in the CNS
Commonly found between neurons and blood vessels in the CNS
Provide structural support
Join parts by their abundant cellular processes
Help regulate the concentrations of nutrients and ions within the tissue
Form scar tissue that fills spaces following injury to the CNS

16. Blood-brain Barrier
The cells that form capillaries (the smallest blood vessels) in the brain are very tightly connected due to astrocytes.
This creates a “blood-brain” barrier that shields brain tissue from chemical changes and blocks entry into the brain from many substances.
This barrier results in selective drug delivery
Antihistamines may not enter the brain so they don’t cause drowsiness
Many drugs needed by the brain may not get into the brain.

17. Types of Neuroglia Ependymal cells Found only in the CNS
Form an outer membrane that covers specialized brain parts (choroid plexus)
Form the inner lining that encloses the spaces within the brain (ventricles) and the spinal cord (central canal)

18. Types of Neuroglia Schwann Cells Found only in the PNS
Forms a myelin sheath around the axons of the neurons
The cytoplasm and nuclei that remain outside the myelin sheath form the neurilemma or neurilemmal sheath

19. Neuron Structure Neurons vary considerably in size and shape
Parts of a neuron:
Cell body or soma consists of
Granular cytoplasm that contains membranous sacs called chromatophilic substance or Nissl bodies.
Nissl bodies are similar to rough endoplasmic reticulum in other cells.

20. Neuron Structure Parts of a neuron (cont): Cell body (cont)
A large spherical nucleus with a conspicuous nucleolus
Neurofibrils are fine threads that will extend in to the axon
Organelles such as
Mitochondria
Lysosomes
Golgi apparatus

21. Neuron Structure Parts of a neuron (cont): Dendrites
Tubular and Cytoplasm-filled
Usually short and highly branched
Usually several per cell body
Together with the cell body, receive nerve impulses
Send nerve impulses toward the cell body

22. Neuron Structure (cont.)
Axons
Usually one per neuron
Conducts nerve impulses away from the cell body and toward another neuron
Arises from a light elevation of the cell body called the axonal hillock
Mitochondria, microtubules, and neurofibrils are in the axon cytoplasm
Originates as a single structure, but may give off side branches called collaterals
Ends may branch into many fine extensions that contact the receptive surfaces of other cells (muscles, glands, other neurons).

23. Neuron Structure

24. Neuron Structure

25. Myelin Sheath & Neurilemma
Myelin sheaths
Enclose the axons of larger peripheral neurons
Produced by Schwann cells
Schwann cells
Wrap tightly around axons (like a bandage around a finger)
Coat axons with many layers of cell membrane that have little or no cytoplasm between them forming the myelin sheath
The part of the Schwann cell containing most of the cytoplasm and the nuclei, remains outside the myelin sheath and makes up the neurilemma or neurilemmal sheath and are found outside the myelin sheath
Gaps between Schwann cells are called Nodes of Ranvier.

26. Myelin Sheath & Neurilemma

27. Nerve Regeneration Peripheral nerves CNS nerves
When damaged, the neurilemma allows the axon to regenerate
CNS nerves
Myelinated by oligodendrocytes which provide a myelin sheath but no neurilemma
When damaged, the axon does not regenerate
The brain contains some neural stem cells (in the hippocampus near the ventricles) which can divide to produce new neurons or neuroglial cells

28. Neuron Classification
Neurons differ in:
Structure
Size
Shape
Length and size of axons
Length and size of dendrites
Number of connections made to other neurons
Neurons can be classified by:
Function
Each type of neuron is specialized to send a nerve impulse in one direction.

29. Neuron Classification – Structure
Multipolar neurons
Have many processes arising from their cell bodies
Only one process of each neuron is the axon
All other processes are dendrites
Most commonly found in neurons whose cell bodies lie within the brain or spinal cord

30. Neuron Classification – Structure
Bipolar neurons
Have only two processes, one arising from each end of the cell body
Each process is structurally similar
One process is an axon and one is a dendrite
Found in specialized parts of the eyes, nose, and ears

31. Neuron Classification – Structure
Unipolar
Have a single process extending from the cell body
The process divides into two branches, each of which function as an axon
One branch (peripheral process) is associated with dendrites near a peripheral body part
The other branch (central process) enters the brain or spinal cord
The cell bodies of some unipolar neurons aggregate in special masses of nervous tissue called ganglia and are found outside the brain and spinal cord

32. Neuron Classification – Structure

33. Neuron Classification – Function
Sensory neurons
Also called afferent neurons
Carry nerve impulses from peripheral body parts into the brain or spinal cord
Have specialized receptor ends at the tips of their dendrites, or
Have dendrites closely associated with receptor cells in the skin or sensory organs
Changes inside or outside the body stimulate the sensory neuron, triggering a nerve impulse
Most are unipolar, some are bipolar

34. Neuron Classification – Function
Interneurons
Also called association or internuncial neurons
Lie entirely within the brain or spinal cord
Multipolar and link with other neurons
Transmit impulses from one part of the brain or spinal cord to another
May direct incoming sensory impulses to appropriate parts for processing and interpreting
The cell bodies of some interneurons aggregate in specialized masses of nervous tissue called nuclei.
Nuclei are similar to ganglia but are located within the CNS

35. Neuron Classification – Function
Motor Neurons
Also called efferent neurons
Multipolar neurons
Carry nerve impulses out of the brain or spinal cord to effectors
Stimulate muscles to contract and glands to release secretions

36. Neuron Classification – Function

37. The Synapse Nerve impulses travel along complex nerve pathways
The junction between two communicating neurons is called a synapse
The neurons are not in direct physical contact
Separated by a gap called a synaptic cleft
Communication along a nerve pathway must cross these gaps

38. The Synapse
The neuron carrying the impulse into the synapse is the sender, or presynaptic neuron
The neuron that receives this impulse at the synapse is the receiver or the post-synaptic neuron
The process of crossing the synaptic cleft with the impulse is called synaptic transmission
A one-way process carried out by chemicals called neurotransmitters

39. The Synapse
The distal ends of axons have one or more extensions called synaptic knobs that contain many small sacs called synaptic vesicles
When a nerve impulse reaches a synaptic knob, some of the vesicles release neurotransmitter into the synaptic cleft
The neurotransmitter diffuses across the synaptic cleft and reacts with specific receptors on the post-synaptic neuron membrane

40. Neurotransmitters Actions of neurotransmitters can be
Excitatory as in turning a process on
Inhibitory as in turning a process off
The net effect of neurotransmitters on a postsynaptic neuron depends on the combined effect of excitatory and inhibitory inputs from presynaptic neurons
There can be from ,000 presynaptic neurons

41. Cell Membrane Potential
The surface of a neuron’s cell membrane is usually electrically charged or polarized when compared to the inside of the cell membrane
Polarization arises from an unequal distribution of positive and negative ions between the sides of the membrane
Very important to the conduction of muscle and nerve impulses
A characteristic change in neuron membrane polarization and return to its resting state is called an action potential and forms a nerve impulse that is propagated or sent along an axon.

42. Distribution of Ions Cells throughout the body have
A greater concentration of sodium ions (Na+) outside
A greater concentration of potassium ions (K+) inside
The distribution of ions inside and outside a cell is determined by channels in cell membranes that let ions in or out
Some are always open
Some can be opened or closed
Can be selective
Let some ions in
Keep other ions out

43. Distribution of Ions
Potassium ions (K+) pass through cell membranes much more easily than sodium ions
This makes K+ a very important contributor to membrane polarization
Calcium ions (Ca2+)are less able to cross the resting cell membrane than either sodium or potassium ions, but have a special role in nerve function

44. Resting Potential
The difference in electrical charge between two regions is called a potential difference.
In a resting neuron, the potential difference between the region inside the membrane and the region outside the membrane is called a resting potential.
Inside the membrane the charge is slightly negative and the potassium concentration is higher
Outside the membrane the charge is slightly positive and the sodium concentration is higher.

45. Resting Potential
Sodium & potassium ions move from where there is a higher concentration to where there is a lower concentration
There is a higher concentration of sodium outside the membrane
There is a higher concentration of potassium inside the membrane
Potassium moves through the cell membrane more easily than does sodium.

46. Potential Changes Nerve cells are excitable
Respond to changes in their surroundings
Some are specialized to detect specific changes
Temperature
Light
Pressure from outside the body
Many respond to neurotransmitters from other neurons
If a membrane’s resting potential changes
The inside of the membrane becomes less negative compared to the outside
The membrane is said to be depolarized

47. Potential Changes
The magnitude of change in a resting potential is directly proportional to the intensity of the stimuli
if the membrane is being depolarized, the greater the stimulus, the greater the depolarization
If neurons are depolarized sufficiently, the membrane potential reaches a level called the threshold potential
Approximately -55 millivolts
If threshold is reached, an action potential results
Action potential is the basis for a nerve impulse

48. Action Potential At threshold potential
Permeability changes at the trigger zone of the neuron being stimulated
Sodium channels open and allow sodium ions to diffuse inward through the cell membrane
This causes the inside of the cell membrane to become more positive
Results in depolarization

49. Action Potential
At almost the same time, potassium channels open and potassium ions diffuse outward through the cell membrane
This causes the inside of the membrane to become more negative again
Results in repolarization

50. Action Potential
The membrane potential may briefly become overly negative
Hyperpolarization
Refractory period
The membrane potential then quickly returns to the resting potential
Remains in this state until the neuron is stimulated again.

51. Action Potential
The rapid sequence of depolarization and repolarization takes about 1/1000th of a second
Results in an action potential
Active transport within the cell membrane maintains the original concentrations of sodium and potassium on either side of the membrane.

52. Action Potential

53. Nerve Impulses
When an action potential occurs in one region of a nerve cell membrane
It causes a bioelectric current to flow to adjacent parts of the membrane
This local current stimulates the adjacent membrane to its threshold level and triggers another action potential
This stimulates the next adjacent region
A wave of action potentials moves down the axon to the end
The propagation of action potentials along a nerve axon constitutes a nerve impulse
See Table 9.1, page 224

54. Impulse Conduction Unmyelinated axons Myelinated axons
Conduct an impulse over its entire surface
Myelinated axons
The myelin sheath would prevent the conduction of a nerve impulse altogether if it were continuous
Nodes of Ranvier between Schwann cells interrupt the myelin sheath
Action potentials happen at the nodes of Ranvier where the axon is exposed
The action potential appears to jump from node to node and is called saltatory conduction
Nerve impulses travel much faster than on an unmyelinated axon

55. Saltatory Conduction

56. Speed of Impulse Conduction
Myelinated neurons conduct faster than unmyelinated neurons
Due to saltatory conduction
The speed of impulse conduction is proportional to the diameter of the axon
The greater the diameter, the greater the speed
A relatively thick, myelinated neuron associated with a skeletal muscle might travel 120 meters per second
A thin, unmyelinated neuron such as a sensory neuron of the skin, might travel only 0.5 meters per second

57. All or None Response
If a neuron responds at all, it responds completely
A nerve impulse is conducted whenever it reaches threshold stimulus in the axon
Threshold stimulus is the strength of stimulation that causes a neuron to reach threshold potential
All impulses carried on that axon are of the same strength
A greater intensity of stimulation doesn’t produce a stronger impulse
Produces more impulses per second

58. Refractory Period For a very short time following a nerve impulse,
A threshold stimulus will not trigger another impulse
The refractory period occurs
The frequency of impulses in a neuron are limited
This insures that an impulse will proceed in only one direction down the axon
Although 700 impulses per second are possible, 100 impulses per second is more common.

59. Excitatory and Inhibitory Actions
Excitatory neurotransmitters
Increase post-synaptic membrane permeability to sodium ions
Bring the membrane closer to threshold
May trigger nerve impulses
Ex: glutamic acid
Inhibitory neurotransmitters
Make it less likely that threshold will be reached
Decrease the chance that a nerve impulse will occur
Ex: serotonin
Some neurotransmitters may be excitatory or inhibitory depending on the receptors in the post-synaptic neuron

60. Neuronal Pools Neurons in the CNS are organized into neuronal pools
Groups of neurons that make hundreds of synaptic connections with each other
Work together to perform a common function
Each pool receives input from neurons
Neurons may be in more than one pool
Each pool generates output
May have excitatory or inhibitory effects on other pools or on peripheral effectors

61. Facilitation
Sub-threshold stimulation of a neuron that increases responsiveness to further stimulation
Neurons in a neuronal pool may receive excitatory or inhibitory input
If the input is excitatory, threshold may be reached and an outgoing impulse may be triggered
If the net effect is excitatory but sub-threshold, an impulse may not be triggered, but the neuron may be more excitable

62. Convergence and Divergence
Occurs when axons from different parts of the nervous system lead to the same neuron
Makes it possible for impulses arriving from different sources to have an additive effect on a neuron
Divergence
Occurs when impulses leaving a neuron pass into several other output neurons
Can amplify or spread the impulse to other neurons

63. Types of Nerves Nerves are bundles of axons
Axons are neuron processes that can be referred to as nerve fibers
Neuron processes that bring sensory information into the CNS are called sensory fibers or afferent fibers
Nerves that conduct impulses into the CNS are sensory nerves
Neuron processes that carry impulses from the CNS to effectors are called motor fibers or efferent fibers
Nerves that conduct impulses out of the CNS are motor nerves
Most nerves contain sensory and motor fibers and are called mixed nerves

64. Types of Nerves

65. Organization of Nerve Fibers
Nerve fibers are separated into bundles in a way similar to muscle fibers
An entire nerve is defined by an outer epineurium
A perineurium separates nerve fibers into bundles called fascicles
An endoneurium surrounds an individual nerve fiber.

66. Nerve Pathways
The routes that nerve impulses follow as they travel through the nervous system are called nerve pathways
The simplest nerve pathway is called a reflex arc
Contains only a few neurons
Constitutes the structural and functional basis for involuntary actions called reflexes
The pathway for a reflex arc follows this sequence:
Begins with a receptor at the end of a sensory neuron
Sensory neuron carries the message to one or more interneurons in the CNS
Interneurons will communicate with motor neurons whose axons pass outward from the CNS to effectors
Effectors (usually muscles or glands) will respond to the message from the motor neurons

67. Reflex Arc

68. Reflexes Automatic responses to changes within or outside the body.
Help maintain homeostasis by controlling many involuntary processes such as:
Heart rate
Breathing rate
Blood pressure
Digestion
Also carry out automatic actions of
Swallowing,
Sneezing
Coughing
Vomiting

69. Patellar Reflex or Knee-Jerk Reflex
An example of a simple reflex
Involves a pathway of only 2 neurons
Sensory neuron
Motor neuron
Striking the patellar ligament just below the patella initiates the reflex
The quadriceps femoris muscle group (attached to the patella by a tendon) is pulled slightly, stimulating stretch receptors in these muscles
These receptors trigger impulses that pass along the axon of a sensory neuron into the spinal cord
In the spinal cord, the sensory neuron makes a synapse with a motor neuron
An impulse is triggered back along the axon of the motor neuron back to the quadriceps
The muscle contracts in response, and the leg extends.

70. Reflex Arc

71. Withdrawal Reflex
Occurs when a person unexpectedly touches a body part to something painful
Example: stepping on a tack
Activates skin receptors and sends sensory impulses to the spinal cord
The impulses pass to the interneurons of a reflex center
The impulses then are directed to motor neurons
The motor neurons transmit signals to flexor muscles in the injured part, and the muscles contract in response
At the same time, the antagonistic extensor muscles are inhibited
The foot is rapidly and unconsciously withdrawn from the painful stimulus
At the same time as the withdrawal reflex, sensory impulses are sent to the brain, and the person may be aware of pain
Reflexes are protective and may limit damage to the body

72. Withdrawal Reflex